Emerging pathogens such as human immunodeficiency virus type 1 (HIV-1) are of central importance to VA Medical Centers across the United States. In fact, the VA is the single largest healthcare provider for adults infected with HIV in this country. Although it has been over 25 years since HIV-1 was first isolated, the virus remains an emerging pathogen worldwide with an average of 6,800 new infections, and 5,700 deaths occurring daily. Historically, vaccination has proven to be one of the most effective tools in the control of viral pandemics. Vaccine development is traditionally driven by naturally occurring protective immune responses. Unfortunately, this strategy has not proven to be successful in the fight against HIV. Moreover, the precise correlates of protection from HIV disease remain elusive. This has recently led to a redirection of the HIV vaccine research priorities. One of the critical areas identified in this effort is to determine why broadly neutralizing antibodies (nAbs) are uncommon and how they can be elicited. The central hypothesis of this renewal application Improving Antibody Responses to Virion Based HIV Vaccines is that native HIV envelope (Env) is not an optimal antigen for the humoral immune system. As such, we believe that the failure of natural infection with HIV and of current vaccine strategies to induce protective antibodies (Abs) is linked to the inability of native Env to readily elicit these types of Abs. Thus it has become increasingly important to identify components of HIV Env that can induce protective antibody (Ab) responses, as well as to identify those components which may be involved in the elicitation of potentially detrimental enhancing antibodies. To this end, we have used Env modified formaldehyde and heat treated virions and pseudovirions as tools to study correlates of Ab mediated protection. During our initial funding period, we have isolated a panel of monoclonal antibodies (mAbs) immunized with Env modified heat treated virions. Interestingly, we have isolated mAbs which possess neutralizing activity and yet which fail to score positive in ELISA based binding assays against virions or HIV specific monomeric proteins. Overall the data presented here specifically demonstrate that Env modified heat treated virion based vaccines can show enhanced antigenicity in vitro and are immunogenic in vivo, and that unique mAbs can be isolated after immunization with these preparations. We hypothesize that these are precisely the types of reagents that can be successfully exploited to identify critical structures on Env that should be targeted and/or avoided in the eventual derivation of HIV vaccines.